The present invention relates to a phase locked loop using a digital phase comparator, and more particularly to a circuit for detecting the lock state of the phase locked loop.
U.S. Pat. No. 4,287,480 issued Sep. 1, 1981 to Swift et al for Phase Locked Loop Out-Of-Lock Detector discloses detection of the out-of-lock condition of a digital phase locked loop during a plurality of states such that single bit discrepancies are ignored. The disclosed phase locked loop detector includes a circuit in the feedback path to determine which mode of the phase locked loop feedback will be used to ignore single bit discrepancies.
U.S. Pat. No. 4,633,488 issued Dec. 30, 1986 to Shaw for Phase-Locked Loop for MFM Data Recording, and U.S. Pat. No. 4,745,372 issued May 17, 1988 for Phase-Locked-Loop Circuit Having a Charge Pump disclose phase locked loops having charge pumps and voltage controlled oscillators.
U.S. Pat. No. 4,929,916 issued May 29, 1990 to Fukuda for Circuit for Detecting a Lock of Phase Locked Loop discloses a circuit having a phase comparator receiving a first signal and a second signal for generating a phase difference signal, a waveform shaping circuit connected to receive the phase difference signal for generating an output signal when the phase difference signal indicates a phase difference exceeding a predetermined value, and a multi-stage counter having a frequency division function and having a reset input connected to receive the output signal of the waveform shaping circuit.